The misspelling of Darth Vader's name probably occurred because this was very early when people weren't sure of the correct way to spell the characters' names, and there wasn't much Star Wars merchandise with Vader's name on it. I have a pin button with that exact same Darth Vader image and misspelling on it.

I remember when they used to spell Obi-wan as OB-1 and I think they did the same thing with the droids, but in reverse. Cee-three-pee-o, Ar-too-dee-too, that sort of thing. But I might might be wrong, I'm very old now and my brain is full of irrelevant nonsense.